Gas flow controlling system

ABSTRACT

The present invention relates to an arrangement for controlling the mass rate of flow of a particular gas to a predetermined level and to a high degree of accuracy. The arrangement is particularly adapted to form part of a gas sampler arrangement. The errors of the prior art gas sampling techniques attributable to changes in temperature, barometric pressure, flow resistance, and pump efficiency are obviated. The arrangement is also well adapted to control the mass flow rate of compressed gases. The arrangement is a three valve system through which the gas flows. The first valve is a pressure regulator valve controlled by the downstream pressure drop through the second valve and set to increase or reduce gas flow as needed to maintain the pressure drop across the second valve constant. The third valve which is downstream of the regulator and the second valve is set to operate under critical pressure ratio conditions. The flowing gas is then exhausted from the system, usually by operation of a vacuum pump or by discharge to the atmosphere. If the three valve arrangement is maintained in a thermostatically controlled, heated enclosure, the gas flows through the three valve arrangement at a constant mass rate, regardless of changes in flow resistances, pump efficiency temperature and barometric pressure of gas entering the system. The present invention relates to a gas flow path adapted to constant mass flow therethrough. The system is particularly well adapted for gas sampling purposes. A close examination of air sampling techniques employed by the art indicates that mass semi-conductor errors of about 5 percent are the rule and that errors thyristor, 10 percent are not at all uncommon. The errors arise from normal temperature and barometric pressure changes in the atmosphere. Measurement uncertainty occurs, particularly in unattended equipment especially in locations where air temperature and clogging of filters are not controlled. For example, a 30* F. temperature change over any 24 hour period is common in many parts of the United States, which difference would cause an accurate constant volume pump operated over the 24 hour period to have a mass sampling through put about 5.5 percent lower at the maximum temperature that at the minimum. If in addition there were barometric changes, e.g. 0.5 inches of mercury over the same 24 hour period, this could add another 1.6 percent difference. In actuality temperature and pressure changes of this daily magnitude occur in about one-third of the reported U.S. cities. Moreover meteorological changes in temperature and pressure associated with storm centers during the winter often double the above values. Clogging of filters and of analytical apparatus by pollutants (usually why the air is being sampled) contribute to sampling errors. Actually changes in flow resistance due to filter clogging or to a shift from one pollutant absorber to another often exceeds the effect of barometric pressure changes. In addition, variations in pump efficiency can have major effect on conventional samplers, most of which depend in some manner upon the performance of the pump. Most continuous sampling methods (for air pollutants) specify that the volume of air sampled be converted to the volume it would occupy at standard conditions of pressure and temperatures, usually 760 mm Hg and 25* C., the conversion has created a set of units for expressing the quantity of air as &#39;&#39;&#39;&#39;standard&#39;&#39;&#39;&#39; volumes. The units used most frequently are the standard cubic foot, the standard liter and the standard milliliter. Despite their expression in volume terms these units correspond exactly to a fixed mass of air and are, in effect, mass units, a standard linter (760 mm Hg and 25* C.) weighs 1.184 grams, a standard cubic foot of air weighs 0.07394 pounds (33.53 grams). Clearly the accuracy of reported measurements for air pollutant content can be no better than the initial measurement of the mass of air sampled. The principal object of the present invention is to provide a system capable of maintaining an essentially constant mass flow rate for the particular gas passing through. A further object of the present invention is to provide an air sampler system wherein the air sample passes therethrough at an essentially constant mass rate of flow. Further objects and advantages of the present invention will become apparent from the description thereof which follows.

United States Patent Steele [73] Assignee: National InstrumentLaboratories, Inc.,

Rockville, Md.

[22] Filed: June 15, 1970 [21] Appl.No.: 45,965

[52] US. Cl... ..l37/334, 137/341 [51] Int. Cl ..Fl6k 49/00 [58] FieldofSearch ..73/421.5 A, 421.5 R,211; 137/334, 501

[56] References Cited UNITED STATES PATENTS 2,917,067 12/1959 Pearl..l37/468 X 2,924,697 2/1960 Thomson ..l37/468 X 3,240,061 3/1966 Bloomet al.... ..73/211 3,395,726 8/1968 Cross et al. 137/468 991,641 5/1911Plantinga.... 137/501 X 3,438,261 4/1969 Collins, Jr... ..73/421 53,530,891 9/1970 Welland 137/613 X 1,503,591 8/1924 Kerr et a1 137/501 X1,643,155 9/1927 Eisenschitz. 73/4215 A 2,339,753 1/1944 Bloom ..l37/5012,534,489 12/1950 Webber et a1. ..73/421 5 3,447,360 6/1969 Laseter......73/42l.5 X 3,495,463 2/1970 Howell ..73/421.5 A

[151 3,653,399 51 Apr. 4, 1972 Primary Examiner-Samuel ScottAttorney-Fidelman, Wolffe & Leitner [57] ABSTRACT The present inventionrelates to an arrangement for controlling the mass rate of flow of aparticular gas to a predetermined level and to a high degree ofaccuracy. The arrangement is particularly adapted to form part of a gassampler arrangement. The errors of the prior art gas sampling techniquesattributable to changes in temperature, barometric pressure, flowresistance, and pump efficiency are obviated. The arrangement is alsowell adapted to control the mass flow rate of compressed gases.

The arrangement is a three valve system through which the gas flows. Thefirst valve is a pressure regulator valve controlled by the downstreampressure drop through the second valve and set to increase or reduce gasflow as needed to maintain the pressure drop across the second valveconstant. The third valve which is downstream of the regulator and thesecond valve is set to operate under critical pressure ratio conditions.The flowing gas is then exhausted from the system, usually by operationof a vacuum pump or by discharge to the atmosphere.

1f the three valve arrangement is maintained in a thermostaticallycontrolled, heated enclosure, the gas flows through the three valvearrangement at a constant mass rate, regardless of changes in flowresistances, pump efficiency temperature and barometric pressure of gasentering the system.

6 Claims, 1 Drawing Figure O TLET GAS FLOW CONTROLLING SYSTEM Thepresent invention relates to a gas flow path adapted to constant massflow therethrough. The system is particularly well adapted for gassampling purposes.

A close examination of air sampling techniques employed by the artindicates that mass semi-conductor errors of about 5 percent are therule and that errors thyristor, percent are not at all uncommon. Theerrors arise from normal temperature and barometric pressure changes inthe atmosphere. Measurement uncertainty occurs, particularly inunattended equipment especially in locations where air temperature andclogging of filters are not controlled. For example, a 30 F. temperaturechange over any 24 hour period is common in many parts of the UnitedStates, which difference would cause an accurate constant volume pumpoperated over the 24 hour period to have a mass sampling through putabout 5.5 percent lower at the maximum temperature that at the minimum.If in addition there were barometric changes, e.g. 0.5 inches of mercuryover the same 24 hour period, this could add another 1.6 percentdifference. In actuality temperature and pressure changes of this dailymagnitude occur in about one-third of the reported U.S. cities. Moreovermeteorological changes in temperature and pressure associated with stormcenters during the winter often double the above values. Clogging offilters and of analytical apparatus by pollutants (usually why the airis being sampled) contribute to sampling errors. Actually changes inflow resistance due to filter clogging or to a shift from one pollutantabsorber to another often exceeds the effect of barometric pressurechanges. In addition, variations in pump efficiency can have majoreffect on conventional samplers, most of which depend in some mannerupon the performance of the pump.

Most continuous sampling methods (for air pollutants) specify that thevolume of air sampled be converted to the volume it would occupy atstandard conditions of pressure and temperatures, usually 760 mm Hg and25 C., the conversion has created a set of units for expressing thequantity of air as standard volumes. The units used most frequently arethe standard cubic foot, the standard liter and the standard milliliter.Despite their expression in volume terms these units correspond exactlyto a fixed mass of air and are, in effect, mass units, a standard linter(760 mm Hg and 25 C.) weighs 1.184 grams, a standard cubic foot of airweighs 0.07394 pounds (33.53 grams).

Clearly the accuracy of reported measurements for air pollutant contentcan be no better than the initial measurement of the mass of airsampled.

The principal object of the present invention is to provide a systemcapable of maintaining an essentially constant mass flow rate for theparticular gas passing through.

A further object of the present invention is to provide an air samplersystem wherein the air sample passes therethrough at an essentiallyconstant mass rate of flow.

Further objects and advantages of the present invention will becomeapparent from the description thereof which follows.

The attached semi-conductor diagrammatically illustrates the flowcontrol system of the present invention.

Briefly stated, the present flow control system constitutes atemperature controlled enclosed gas flow path having three valvesthrough which the gas flows in sequence.

The first valve is a differential pressure regulator which automaticallycontrols the open area of the valve orifice (and therefore the flowrate) in accordance with pressure differential. Here the differentialpressure regulator operates according to the pressure differentialacross the valve orifice of the second valve in the flow path. Thepressure in the flow path segment between the second and third valve ofthe sequence is applied back at the differential pressure regulator tocontinuously and automatically adjust the regulator orifice to maintain,essentially constant, the pressure differential or drop across thesecond valve. After tranversing the flow path segment between the secondand third valve, the flowing gas exits through the third valve.

The second and third valves are adjustable, but during any actualoperation run with a given gas, gas temperature, etc. their settingremains unchanged. in addition the valve orifice setting of the secondvalve must be above the critical pressure ratio setting and the settingof the third valve always must be at a valve orifice setting below thecritical pressure ratio setting for gas flow through the system. For airsampling purposes a suction pump downstream of the third valve will drawenough air through the flow path. To achieve the desired pressure dropin the flow path, the pump should be adequate to keep the absolutepressure downstream of the third valve below about half the pressureupstream of the third valve, a pressure ratio below about 0.45 beingpreferred, this being clearly below critical pressure ratios.

Overall the system continually self-adjusts to maintain the gas flowrate at an essentially constant mass flow rate regardless of changes intemperature and pressure of the incoming gas. In non-mathematical terms,use is made of different flow vs. pressure relations in the second andthird valves. At the second valve, the mass flow rate therethrough isproportional to the square root of the upstream density, andconsequently to the square root of the upstream pressure. At the thirdvalve, the mass flow rate therethrough is directly proportional to theimmediate upstream pressure. If while system is operating inequilibrium, the pressure upstream of the third valve is increased bysome unspecified disturbance, the pressure upstream of the third valvewill soon be restored to its former equilibrium value by action of thedifferential pressure regulator. This restoration takes place becausethe mass flow through the third valve has increased in proportion to theincrease in the pressure, while the mass flow through the second valvehas increased only in proportion to the square root of the pressureupstream of the second valve, with the difference in flowcharacteristics changing the pressure differential across the secondvalve and in turn the position of the regulator valve orifice until thelatter reaches whatever setting reestablishes the equilibrium flowcondition through the second and third valves. Any pressure drop in theentering gas will decrease the differential, causing the regulatororifice to open more for increased flow. Temperature changes in the gasshould not occur because of thermal regulation. In consequence, gasflows through the system at a constant mass flow rate.

Referring now to the drawing it may be seen that the gas flow controlsystem of the present invention may be assembled from standardcomponents available to the art, namely valves, tubing, gages, etc.

Air, or any other gas enters the gas flow control system 10 at inlet 12into inlet section 14 of the flow path, the entering gas present in thissection 14 or section A has an absolute pressure which can bedenominated as P The entering gas passes from inlet section 14 through aregulator valve 16 (the first valve) into the second section 18 of theflow path, wherein it has a (lesser) pressure, denominatable as P,,. Theas then passes through the second valve 20, into section 22 of the flowpath, wherein the gas now has a still lower absolute pressuredenominated as P Thereafter the gas passes through valve 24 to enter thefinal portion 26 of the flow path, wherein it now has an absolutepressureP The gas then exits to some suitable outlet 29 or alternativelyto outlet 28 by way of a vacuum pump 50 sized and operated so that theratio of F ll, is less than the critical pressure ratio for the gas. inpractical operation pump 50 is sized to ensure a P /P ratio less thanabout 0.45 to be certain that flow through valve 24 at critical pressureratio is unaffected by wear on the vacuum pump, leakage, etc.

Gage 30 (which may be a standard Bourdon tube gage is connected tosection 22 of the flow path. Among other things gage 30 may aid insetting valves 20 and 24 for each particular installation, and desiredmass flow rate for the gas. Assuming, as is usually the case, that thevalue of P is known, then the reading of gage 30 will advise whether Pis high enough for P to be less than 0.45 P

Flow path section of portion 22 is connected back to pressure regulator16 by a line 32, so that the diaphragm 46 of pressure regulator 16 isoperated with reference to the pressure drop across valve 20, namely P PThe differential pressure regulator, 16, may be any one of a number ofcommercial regulators which are normally used for the purpose ofcontrolling the differential pressure across a downstream restriction,here valve 20. The drawing depicts the principle of operation of atypical commercially available regulator which has been usedsuccessfully. In this regulator the base 40 and the bonnet 42 areseparated internally by a flexible spring loaded diaphragm 46, thespring being shown as 48. Line 32 transmits the pressure inside of flowpath section 22 to the inside of bonnet 42. The pressure inside the baseportion of 40 regulator 16 is P,, the same pressure as in flow pathsection 18. The gas flowing through the second valve develops adifierential pressure so that P is higher than P and this pressuredifferential (P P is applied to diaphragm 46 causing it to move upwardagainst the restraining forces of spring 48. Diaphram 46 is furtherconnected to the valve stem 60 so that whenever the difference betweenP,, and P increases or decreases the movement of diaphram 46 restrictsor opens the valve orifice of pressure regulator 16. Thus partialclosure of the valve of pressure regulator 16 in turn limits gas flowthereby reducing the pressure differential (P b P across valve 20. ifthe entering gas pressure drops the regulator 16 will operate to permitmore gas to enter thereby increasing the pressure differential (P P Animportant aspect of the present invention is that the entire flow pathof system 10 is inside a housing 100 and therein is thermostaticallycontrolled to some moderately elevated temperature by operation ofelectrical heater 102. The entire flow path and the flowing gas isheated to the constant temperature level inside chamber 100, andmoreover this temperature level is somewhat higher (e.g. 120 F.) thanthe maximum temperature level which may be expected for the inlet air orwhatever gas enters inlet 12.

Not shown on the drawing is the ancillary equipment which normally wouldbe associated with the mass flow rate controller of the presentinvention, e.g. flow meters, filters, etc. An actual air samplinginstallation would also have test equipment such as reaction tubesthrough which the air bubbles to measure gaseous pollutants like sulfurdioxide, or carbon dioxide; or possibly a moving filter through whichthe air is passed to measure presence of solid particles like ash ordust. These and other test equipment would be positioned in advance ofinlet 12 so that all of the air sampled first passes therethrough thenthrough the mass flow controller system. in a typical exemplary airsampling system the absolute pressures of the ambient air is about 30inches t 1.5 inches Hg. The pressure loss on passage throughauxialiaries like flow meters and the test equipment reduces the airpressure and makes P,, about 28.5 inches 2.0 inches Hg, and P about 21.5inches Hg, P about 18 inches Hg, and P,,-about 8 inches Hg. Commerciallyavailable vacuum pumps can readily attain vacuums in excess of 22 inchesHg.

An alternative mode of operation of the mass rate flow control system ofthe present invention is the sampling of compressed gases, inlet 12 thenbeing connected through a flow meter to whatever equipment is testingthe compressed gas. If the initial gas pressure is high enough theterminal flow path section 26 exhausts through outlet 29 to atmosphere.

A related use for the present system is where a constant mass flow rateof gas is employed downstream of the control system rather thanupstream, as for example, where the gas is intended for the carrier gasin gas chromatography measurement. The compressed gas may flow throughoutlet 29 to the chromatography equipment.

An almost static alternative use also contemplated for the presentsystem involves appreciation that gage 30 taps into an adjustable sourceof constant sub-atmospheric pressure. Once cascade, the system iscalibrated it may be employed to test system barometric altimeters.

Repeated allusion has been made as to how the present system providesessentially for a constant mass flow rate of gas passing therethrough asif this characteristic were self evident. For further understanding ofthe invention the following explanation is presented.

Certain reasonable assumptions are made in terms of the flow controlsystem;

l. The ratio P zP is sufficiently large that the flow through the secondvalve 20 follows the flow equation applicable to non-critical floworifices.

2. The ratio P zP is sufficiently small that flow through the thirdvalve 24 follows the flow equation applicable to a critical flow nozzle,i.e. sonic velocity is attained in flow therethrough.

3. The effective molecular weight of the gas and the specific heat ratioof the gas flowing through the system are reasonably constant.

4. The temperature of the flowing gas and of all components in thesystem is held constant and in particular is the same at the valves(through the thermal regulated action of heater 102 in chamber 5. Theregulator valve 16 is an ideal downstream differential pressureregulator which holds P P constant regardless of changes in its inletpressure P,,. Commercially available regulator valves 16 are quite goodenough even to be considered as ideal pressure regulators over thenarrow pressure differential ranges within which air sampling systemsoperate.

Assumption 1 may be written in e uation form as:

M bc bc pb( b P0) Where:

M is the mass flow rate C is the low coefficient of the valve, 20 A isthe area of that portion of the port opening of the valve, 20, which isopen when valve 20 is operating.

p,, is the gas density in section 18 P,, is the absolute pressure inSection 18 and P is the absolute pressure in Section 22. Since thedensity, p,, can be replaced by P,,/RT,,, Eq. 1 can be written as:

Where:

R is the universal gas constant, and T is the absolute temperature ofthe gas in Section 18. Some comment is necessary in regard to assumption2, above, i.e. P zP is small and that valve 24 operates at criticalpressure ratios, this holds l constant since under these conditionspressure changes downstream of valve 24 cannot be transmitted upstreamof the valve to flow path segment 22.

Under critical pressure ratio conditions:

Where:

7 is the ratio of the specific heat at constant pressure to the specificheat at constant volume.

When the condition of Eq. 3 is met, a decrease in the downstreampressure, P,,, does not increase the mass flow rate of gas through thevalve, 24.

Using published values of y, the critical pressure ratio, P /P, isapproximately 0.49 for most monatomic gases, 0.53 for most diatomicgases (including air), and 0.54 for most triatomic gases. In practicaloperation, l /P is kept below about 0.45 to be certain that the massflow rate is unaffected by changes in P,,. The simplified flow equationfor mass flow through the valve, 24, is given as follows:

ul rrI r By assumption 4, that the temperature is constant with time andis the same throughout the system, the subscripts, b and c, can bedropped from T, and T can be treated as a constant.

Equations 2 and 4 are combined to give:

2P P P cd cd c Che hr By Assumption 5, P, his a constant and may bereplaced by H. Then P, H P This gives:

This equation shows that for fixed valve openings, A and A P, is afunction of constants only. Therefore P is a constant. By Eq. 4, if P isa constant, then M, the mass flow rate is 3 constant.

What is claimed is:

l, A system for passing gas therethrough at an essentially constant massflow rate comprising:

an enclosed gas flow path having therein three open valves through whichthe gas flows in succession, the first valve being a differentialpressure controller valve controlled by the pressure differentialupstream and downstream of the second valve to maintain said pressuredifferential essentially constant, the second valve being set for gasflow therethrough at above the critical pressure ratio, the third valvebeing set for flow therethrough under critical pressure ratioconditions;

and means for maintaining said flow path and the gas flowingtherethrough at a predetermined fixed temperature level.

2. A system as in claim 1 wherein a vacuum pump is disposed downstreamof the third valve to create the desired pressure differentials alongthe length of said enclosed gas flow path.

3. A system, as in claim 1, wherein the second and third valves areadjustable.

4. A system as in claim 3 wherein a gauge is provided to indicate thegas pressure in the flow path segment between the second and thirdsemi-conductor semi-conductor 5. An air sampler comprising a system, asin claim 4 through which an air sample is flowed and pollutantmeasurement means disposed in the air sample flow path ahead of thedifferential pressure controlling valve.

.6. An air sampler, as in claim 5, wherein a vacuum pump is disposeddownstream of the third valve to create the desired pressuredifferential along the length of said gas flow path, and wherein theflow path is thermally regulated to maintain a predetermined temperaturelevel which is above any ambient air temperature levels.

UNITED STATES PATENT ormcr, CERHMCATE Ci QQRMEQTWN Patent No. 3, 653,399 Dated April 4, 1972 Irwen Dale I. Steele It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as SLIOWZJ. bclfowz Column 1, line 6, change ascornx'iuctor to samplingline 7, change t1 istor to of and omit thecomma-F line 17, after the wo;;;d temperature, change that to than--line 58, change semiconductor to drawing.

Column 2, line 56, change to gas,

Column 3, line 52, delete the second "a" from auXiliaries-..--

line 74, delete cascadeline 75, change system to dead end.

Column 4, Equation 3, change Pd to P Column 6, Claim 4, changesemiconductors to valves In Claims 2 through 6 the clause "as in claim"is inconsistently punctuated, In Claim 2 the clause has no comnzassurrounding it; in Claim 3 it is surrounded by commas; in Claim 4 thereare no commas; in Claim 5 there is a comma before the clause but noneafter; and in Claim 6 the clause is enclosed in commas,

Signed and sealed this 15th day of M.

% Shah J I'LFLETUHML ROBERT GOTTSGI L LLK ting; Officer Commissioner ofPatents 1 U GOVERNMENT PRINTING OFF'iCE: I969 O3fi6-334

1. A system for passing gas therethrough at an essentially constant massflow rate comprising: an enclosed gas flow path having therein threeopen valves through which the gas flows in succession, the first valvebeing a differential pressure controller valve controlled by thepressure differential upstream and downstream of the second valve tomaintain said pressure differential essentially constant, the secondvalve being set for gas flow therethrough at above the critical pressureratio, the third valve being set for flow therethrough under criticalpressure ratio conditions; and means for maintaining said flow path andthe gas flowing therethrough at a predetermined fixed temperature level.2. A system as in claim 1 wherein a vacuum pump is disposed downstreamof the third valve to create the desired pressure differentials alongthe length of said enclosed gas flow path.
 3. A system, as in claim 1,wherein the second and third valves are adjustable.
 4. A system as inclaim 3 wherein a gauge is provided to indicate the gas pressure in theflow path segment between the second and third semi-conductorsemi-conductor
 5. An air sampler comprising a system, as in claim 4through which an air sample is flowed and pollutant measurement meansdisposed in the air sample flow path ahead of the differential pressurecontrolling valve.
 6. An air sampler, as in claim 5, wherein a vacuumpump is disposed downstream of the third valve to create the desiredpressure differential along the length of said gas flow path, andwherein the flow path is thermally regulated to maintain a predeterminedtemperature level which is above any ambient air temperature levels.